Copy quality is sensitive to the properties of the copy sheet, such as the roughness and thickness level of the sheet, in marking technologies such as electrostatographic reproduction of images, e.g. electrophotography, and laser printing, as well as thermal ink jet, and thermal transfer reproduction of images. Distinct variations exist as to the levels of roughness and thickness found in individual copy sheets, such as paper, (hereinafter referred to as "substrates") that are used in a substrate marking machine. Various machine parameters are affected by the levels of roughness and thickness of a particular substrate to be processed through the machine. These machine parameters can be adjusted according to varying substrate properties, so as to optimize their functionality within the machine, as well as to achieve optimum output copy quality.
For example, an ideal goal in electrophotography is to have the correct amount of toner deposited onto a copy sheet on a continuous basis. With poor development control two situations occur. First, concerning a variability of toner quantity applied, too little toner creates lighter images, where too much toner creates darker images and may cause toner to appear in non-image areas. Second, concerning the machine, too much toner development causes excess toner waste which both increases the expense of running the machine and wears machine components at a faster pace. Replacement or repair of these components is thereby required on a more frequent basis. The need for precise toner control is intensified in color development systems where individual color images are superimposed on each other to create the full color image.
A rougher substrate contains a greater surface area than a smoother substrate, and may therefore require application of a higher developed toner mass per unit area to get the same dark, uniform, sharp copy quality. Additionally, if a greater amount of toner is required on a rougher substrate, the developer voltage must be increased, as well as the fuser set temperature, and/or the fuser dwell time to assure that the toner is completely fused to the sheet. A substrate having a greater basis weight or thickness (the terms hereinafter to be used interchangeably, based on their linear relationship with each other) escalates the need for higher fuser settings in order for the toner to be sufficiently fused to the substrate. Under-fused toner on a substrate creates the possibility of smears, streaks or blurred copies. An increased voltage may also be required for the corona generating devices associated with both the charging element, as well as the transfer element, for a rougher and/or thicker substrate.
Uniformly increasing machine parameters such as the fuser set temperature, fuser dwell time, or voltage level to the charging device, transfer device or developer may eliminate certain copy quality problems on rougher and thicker substrates, however, other negative factors are thereby created. For example, increased wear on these affected components causes the need for more frequent component repair or replacement. Furthermore, the increased power required to run the component at these higher settings results in increased energy consumption and cost. Machines that can optimize copy quality as well as internal processing operations will, of course, have a tremendous competitive edge.
It is known in the electrical graphic arts to use light sensors for measuring and consequently monitoring the density of a powderous or liquid substance. One such sensor is a developability sensor, also known as a densitometer, used to monitor the "Developed Toner Mass Per Unit of Area," referred to as DMA. A densitometer acts, generally, when toner is illuminated with a collimated beam of light from an infrared light emitting diode (LED), to measure the level of specularly reflected light therefrom. In the case of DMA measurement, toner development onto a substrate correspondingly increasingly attenuates the intensity of the light specularly reflected. The attenuation is the result of either absorption of the incident light in the case of black toners, or by scattering of the incident light away from the specular reflection angle, as in the case of colored toners. Thus, at a high DMA quantity, there is only a very small specular signal, and at a low DMA quantity, there is a higher specular light signal. Examples of densitometers are described in, for example, U.S. Pat. No. 5,053,822, 4,553,033, 4,950,905, and 4,989,985, and are hereby incorporated herein by reference.
Densitometers monitor DMA quantity on a sheet, however, they do not take into account those other machine components and parameters which can be adjusted based on varying roughness and thickness levels of a substrate, in order to optimize their functionality within the machine as well as to achieve optimum output copy quality. Furthermore, the densitometer requires one or more sheets to have run through the system before the DMA quantity on the sheet can be detected and then adjusted. Thus, toner density cannot be monitored continuously on an individual sheet basis.
Sensors for determining sheet basis weight and sheet thickness are also known. U.S. Pat. No. 5,138,178 to Wong et al. describes a non-contact sensor for sensing sheet basis weight, the relevant portions of which are hereby incorporated herein by reference. Wong et al. describes a sensor to determine the sheet basis weight by emitting electromagnetic energy or light from a source of energy, such as an LED, and sensing the amount of energy transmitted through the paper. The amount of energy transmitted through the paper is proportional to the basis weight of the paper, which is directly proportional to its thickness, assuming compositional similarity. U.S. Pat. No. 4,937,460 describes another method for determining the thickness of a sheet, which relies on the contact of the sheet by a receiver or similar element to provide a variable analog signal indicative of paper thickness. These references do not, however, take into account the roughness of the substrate, and are therefore incapable of accommodating adjustments to machine components and parameters that may be necessary for optimum copy quality and machine performance based on varying levels of roughness of individual substrates that pass through the machine. For example, thin sheets that have a high roughness level may require different machine operation settings than, for example, a thin sheet with a very smooth surface; and a thicker sheet having a very rough surface may require different settings than a thicker sheet with a smooth surface.
In response to these problems, a need exists to determine the properties of roughness and thickness of a substrate to be processed through a marking machine, and then correspondingly adjusting the machine parameters that are affected by the varying levels of these substrate properties.
As a result, the present invention provides a solution to the described problems and other problems, and also offers other advantages over the prior art.